1. Field of the Invention
The present invention relates to a developing apparatus which visualizes a static latent image formed on an image carrier with a coloring material of toner, more particularly, to a developing apparatus using a one-component developer as toner.
2. Description of the Related Art
An image forming apparatus which has adopted an electrophotographic method used for copying machines, printers, etc. respectively is provided with a developing apparatus composed so as to form a static latent image on a surface of a photosensitive member, which is a carrier for a latent image, and to then supply a developer such as toner (a coloring material) to the photosensitive material so that the toner is adhered thereon selectively to visualize the latent image.
In the above developing apparatus, the toner image obtained by developing the static latent image formed on the photosensitive member is transferred onto a transfer material such as a sheet, etc. After this, part of toner which is not used in a transfer process remains on the surface of the photosensitive member. This unnecessary residual toner must be removed from the surface of the photosensitive member to enable the next image forming to be performed continuously. The developing apparatus is thus provided with a cleaning device for removing such residual toner from the surface of the photosensitive member after completion of the transfer process. The unnecessary residual toner removed by the cleaning device is then collected in a toner container provided in the cleaning device.
The image forming apparatus provided with the developing apparatus as described above reduces the space for installing process means for forming an image around the photosensitive member, while at the same time, image forming apparatus is becoming more and more compact in size. Thus, such recent trend toward smaller-sized image forming apparatuses necessarily demands a smaller developing apparatus.
Specifically, the developing apparatus as described above is provided with a magnetic brush-type developing roller used for carrying a two-component developer composed of toner and magnetic carrier to a developing area, and which is positioned so as to correspond to the photosensitive member using a magnetic force to collect a residual of the developer in a developing tank after a developing process is ended. For this purpose, new toner is constantly supplied thereby to control the rate of toner included in the developer, that is, the toner density and to stabilize the developing process.
Generally, in the above method, that is, in the magnetic brush type method, the magnetic carrier is included more than the toner in the developer. This makes the developing tank containing the developer larger in capacity, resulting in the enlarging of the comprehensive size of the developing apparatus. In this case, the toner density must also be controlled properly, and the developing apparatus needs a stirring member or the like for stirring the toner in the developer so that the charging state of the toner is fixed. And, since a plurality of such stirring members are provided, it has been difficult to reduce the size of the developing apparatus.
On the other hand, there has been proposed a developing apparatus using toner which is a one-component developer containing no magnetic carrier. The developing apparatus is already in practical use. In the developing apparatus using such one-component toner, there is no need to control the toner density. Since no magnetic carrier exists, the capacity of the developing tank can be reduced and the developing apparatus itself can be miniaturized. Accordingly, such a developing apparatus is also excellent in simplicity for maintenance, etc. In other words, in the case of such a developing apparatus, there is no need to replace a degraded developer, especially a developer including degraded magnetic carrier. No maintenance such as developer replacement is thus needed.
Furthermore, only supply of toner is required; neither detection of toner density nor a controlling means for detecting such toner density is needed. The developing apparatus can thus be easily controlled. Specifically, in the developing apparatus using a one-component toner, toner is supplied only as needed.
For example, as shown in FIG. 1, a developing apparatus 4 used to visualize a static latent image formed on a photosensitive member 1 is disposed so as to be opposed to the photosensitive member 1, which is an image carrier. The developing apparatus 4 is provided with a developing roller 41 rotatable so as to be opposed to, the opening of the developing tank 40 containing toner 10, which is a one-component developer. The portion of the developing roller 41 exposed at the opening of the developing tank 40 is disposed, for example, so as to come in contact with the photosensitive member 1. This contact area is used as a developing area.
The developing roller 41 is used to carry one-component toner 10 on its surface and feed the toner to the developing area opposed to the photosensitive member 1. After a developing process is ended, toner which is not used in the developing process is fed and collected into the developing tank 40. Since the collected toner must be removed at once from the surface of the developing roller 41, the developing apparatus is provided with a feed roller 42 disposed so as to be pressed against the developing roller 41. Toner carried on the surface of the developing roller 41 is scraped off and new toner 10 is fed by the feed roller 42 onto the surface of the developing roller 41.
The one-component toner 10 fed by the feed roller 42 is sucked onto the surface of the developing roller 41. In order to restrict the amount of the toner 10 to be stuck on the surface of the developing roller 41, the developing apparatus is also provided with a toner restricting member 43, disposed so as to be pressed against the surface of the developing roller 41. Toner passing the toner restricting member 43 is restricted so that a fixed amount is fed to the developing area opposed to the photosensitive member 1 as described above. The toner is then stuck on the surface of the photosensitive member 1 according to the static latent image formed thereon. Thus, the latent image is developed.
Usually, a developing bias voltage Va is applied to the developing roller 41 to perform the developing process properly. This developing bias voltage is set to a value that can cause toner to be stuck on the static latent image and not to be stuck on the background area (other than the latent image) of the photosensitive member.
In order to charge one-component toner 10 held on the developing roller 41 to a predetermined potential, and to a predetermined polarity, the toner restricting member 43 pressed against the surface of the developing roller 41 is located at the downstream side of the rotational direction of the developing roller 41. In addition, a restricting voltage Vb is applied to the toner restricting member 43 so as to charge one-component toner 10 to a predetermined polarity. Consequently, one-component toner, when passing the toner restricting member 43, is kept at a fixed amount, charged to a predetermined potential at a predetermined polarity, and fed to the developing area.
Since the developing apparatus is composed as described above, a one-component developer (toner) is applied onto the developing roller and fed to the developing area. Toner is thus stuck on the static latent image formed on the photosensitive member, so that application of toner on the background other than the latent image is prevented, thereby enabling the developing process to perform normally.
In recent years, processings of copying machines and printers are getting faster and faster as described above, while at the same time, there has been a strong demand toward smaller-sized devices. According to such the recent trend, a technology has also been required to secure an image visualizing performance properly in the developing area of each developing apparatus.
In other words, in order to cope with faster processings of an image forming apparatus, it has been derived to feed a developer effectively in its developing apparatus. There has also been a demand for a developing apparatus using one-component toner so that the developing apparatus itself is miniaturized corresponding to the trend toward a smaller-sized image forming apparatus.
This is why the prior art developing apparatus controls the developing roller, the voltage supplied to the developing roller, etc. as shown in FIGS. 5A, 5B, 6A and 6B to stabilize the developing process and feed one-component toner effectively. For example, in response to the start of an image forming operation, application of a developing bias voltage to the developing roller and a supply voltage to the toner restricting member is controlled synchronously with the timing for turning on the developing roller drive motor, so as to be on when the drive motor is on as shown in FIG. 5A.
Otherwise, as shown in FIG. 6A, a fixed time after the developing roller begins rotating, the application of the developing bias voltage and the supply voltage to the toner restricting member is turned on.
Consequently, the developing process is performed stably using toner charged in a predetermined state according to the rotation of the photosensitive member 1. In addition, after the end of the developing process, the developing apparatus is controlled so that the developing bias voltage or the like is turned off synchronously with the stop (OFF) timing for the developing roller, or the developing roller is turned off a fixed time after the developing bias voltage or the like is turned off, as shown in FIG. 5B and FIG. 6B.
As described above, a predetermined voltage is supplied to the developing roller and the toner restricting member respectively, then one-component toner is applied onto the surface of the developing roller and adjusted so that a fixed amount of toner is supplied into the developing area at a fixed charging potential. When an image forming operation is started, however, toner may blow out and scatter while passing the toner restricting member according to the rotation of the developing roller begins before a developing bias voltage is supplied to the developing roller, for example, as shown in FIGS. 5A and 5B. In addition, when an image forming operation ends, if the developing bias voltage is turned off first, toner may scatter before the developing roller stops.
This is because it is difficult to hold the charging potential of toner at a fixed value, since one-component toner is sucked and stuck on the developing roller, for example, statically. Thus, the toner which cannot be applied on the developing roller so statically blows out and scatters when the developing roller begins rotating. As a result, the image quality is degraded in the initial stage of the developing, for example, just after an image forming operation is started. In addition, after the developing process ends, toner which cannot be held on the developing roller is not collected, causing scattering of toner.
Such a problem appears more pronounced when the developing roller is rotated faster, thereby feeding much more toner while the developing apparatus is operated faster. In other words, as the developing roller is rotated faster, the amount of toner scattered also increases.